1. Field of the Invention
This invention relates to a novel compound having an antitumor activity and a process for preparing this compound.
2. Description of the Background
Camptothecin is a penta-cyclic alkaloid isolated from barks, roots, fruits, or leaves of camptotheca acuminata. This compound is known to exhibit an antitumor activity because of its capability of inhibiting nucleic acid synthesis. According to the results of clinical tests conducted in the United States, however, the compound was found to have a problem in view of safety, and its research and development as a medicine have been discontinued. Thereafter, research on derivatives of camptothecin possessing better activity and reduced toxicity has been undertaken worldwide. However, no report has surfaced so far on the derivative with satisfactory clinical results.
The scarce solubility of camptothecin in water is another problem of this compound in administering it as a medicine. A method of opening the lactone ring and converting it into the sodium carbonate is known as one of the means for making camptothecin water-soluble. The product obtained by this method, however, exhibits a very reduced antitumor activity. The development of a water-soluble camptothecin derivative with the lactone ring being retained as is has therefore been desired.
The present inventors have conducted extensive studies for the purpose of obtaining camptothecin derivatives with more excellent activity and higher safety, as well as excellent characteristics required for a drug to be administered, and found that hexa-cyclic compounds obtained by the addition of a water-soluble ring to camptothecin had characteristics superior to camptothecin. This finding has led to the completion of this invention.
Accordingly, an object of this invention is to provide a hexa-cyclic compound represented by the following general formula: 
wherein R1 and R2 individually represent a hydrogen atom, a hydroxyl group, a C1-6 alkyl group (xe2x80x9cC1-6 alkyl group means an alkyl group having 1 to 6 carbon atoms. Hereinafter defined in the same manner.) which may contain a halogen manner.) which may contain a halogen atom, a nitro or cyano group, a C1-6 alkenyl group, a C1-6 alkynyl group, a C1-6 alkoxyl group, a C1-6 aminoalkoxyl group, a halogen atom, a nitro group, a cyano group, a mercapto group, a alkylthio group, an amino group which may contain a protective group, a C1-6 aminoalkyl groups which may contain a protective group or a C1-6 alkyl group at the amino-position, a C1-6 aminoalkylamino group which may contain a protective group or a C1-6 alkyl group at the amino-position, a C1-6 alkyl group with a heterocyclic ring which may contain a C1-6 alkyl, C1-6 alkoxyl, amino, halogeno, nitro, or cyano group, a carbonyl with a-heterocyclic ring which may contain a C1-6 alkyl, C1-6 alkoxyl, amino, halogeno, nitro, or cyano group, a C1-6 alkylamino group with a heterocyclic ring which may contain C1-6 alkyl, C1-6 alkoxyl, amino (which may contain a protective group), halogeno, nitro, cyano or a protective group, an amino-heterocyclic group which may contain a protective group or a C1-6 alkyl group at the nitrogen atom of the heterocyclic ring moiety or at the amino position, a heterocyclic-amino group which may contain a protective group or a C1-6 alkyl group at the nitrogen atom of the heterocyclic ring moiety or at the amino position, or a carbamoyl group which may contain a protective group or a C1-6 alkyl group; R3 represents a C1-6 alkyl group; R4 represents an amino group which may contain a protective group, a quaternary trialkyl ammonium such as xe2x80x94N+ (CH3)3, a C1-6 alkylamino group which may contain a protective group, a C1-6 aminoalkyl group which may contain a protective group, a C1-6 alkylaminoalkyl group which may contain a protective group, a sulfonic acid group, or a carboxyl group; Z represents an oxygen atom, a sulfur atom, CR5R6, wherein R5 and R6 individually represent a hydrogen atom or a C1-6 alkyl, or Nxe2x80x94R7, wherein R7 stands for a hydrogen atom, a C1-6 alkyl. group, a C1-6 aminoalkyl group which may contain a protective group, a C1-6 aminoalkyl group which may contain a protective group, a C1-6 alkylaminoalkyl group which may contain a protective group, or a protective group for the amino group; and m and n individually represent 0, 1 or 2.
Other objects, features and advantages of the invention will hereinafter become more readily apparent from the following description.
Preferred examples of groups represented by R1 or R2 in formula (I) are C1-3 alkyl, C1-3 alkenyl, hydroxymethyl, hydroxyl, C1-3 alkoxyl, halogen, nitro, amino, C1-3 alkylamino, cyano-C1-3 alkyl, aminomethyl, dimethylhydrazino, morpholine-1-yl, piperidine-1-yl, and the like.
Ethyl group and the like are given as preferable groups for R3.
Amino group, C1-6 alkylamino, amino-C1-6 alkyl, C1-6 alkylamino-C1-6 alkyl groups are given as preferred groups for R4. Among these, especially preferable are methylamino, dimethylamino, aminomethyl, ethylamino, diethylamino, aminoethyl, methylaminomethyl, dimethylaminomethyl, hydroxy ethylamino and the like.
When substituent R4 and substituent R1 or R2 of compound (I) are suitably selected, the compound possesses sufficient water-solubility and a suitable degree of lipophilicity, and thus exhibits excellent properties. The lipophilicity is a parameter of cell membrane permeability of the compound, and the improved cell membrane permeability promises cytotoxicity against cancerous cells.
Examples of preferable combination of R4, R1 and R2 include such that:
R4 is NH2, NHCH3 or N(CH3)2, and
R1 or R2 is CH3 or C2H5 
Among them, most preferred is a combination where R4 is NH2, R1 is 4-methyl and R5 is 5-fluoro.
When R4 is NH2, and R1 or R2 is OH, only an insufficient cell membrane permeability is obtained, and therefore, it is preferred that the compound be converted to a pro-drug, in other words, the OH group is converted to xe2x80x94Oxe2x80x94Y or xe2x80x94Oxe2x80x94COxe2x80x94Y. Here, Y is amino, dialkylamino, dialkylaminoalkylamino, aminoalkyl, alkylaminoalkyl, dialkylaminoalkyl,. heterocyclic group which may contain one or more C1-6 alkyl, C1-6 alkoxyl, amino, halogeno, nitro or cyano group, or alkyl group with heterocyclic ring which may contain one or more C1-6 alkyl, C1-6 alkoxyl, amino, halogeno, nitro or cyano groups.
A quaternary-trialkyl ammonium such as xe2x80x94N+ (CH3)3 is also preferred as R4.
Methylene group, oxygen atom, sulfur atom, imino(xe2x80x94NHxe2x80x94), alkylimino(xe2x80x94N(alkyl)xe2x80x94), and the like are given as preferable groups represented by Z.
As preferred protective groups for amino group, given are formyl, acetyll, trithyl, tert-butoxycarbonyl, benzyl, p-methoxybenzyloxycarbonyl, and the like.
Given as preferred examples for heterocyclic groups are 4-7 membered rings having one or more nitrogen atoms which may contain one or more oxygen atoms or sulfur atoms, such as azetidine, piperazine, morpholine, pyrrolidine, piperidine, imiazole, thiazole, oxazole, pyridine, and the like. Among them, those having 5 or 6 membered rings such as pyrrolidine, piperidine, piperazine, morpholine and the like are especially preferred.
Among the compounds of formula (I), those having a six-membered ring for the A-ring, which are represented by the following formula (IA) is particularly preferable. 
Furthermore, among the compounds of formula (I), those in which the asymmetric carbon at 9 position of the F-ring takes the S-type configuration are preferable from the aspect of medicinal activity.
The compounds of the present invention can be prepared according to the process exemplified by the following reaction scheme. 
According to the above process scheme, an aminoketone compound (2) and a pyranoindolizine compound (3) are condensed by the Friedlaender reaction to produce the compound (I).
Aminoketone compounds (2) are known compounds and can be readily prepared according to the methods known in the art. The conditions of this condensation ring-closing reaction of compounds (2) and (3) can be suitably selected from the conditions, wherein the reaction is conducted at room temperature or an elevated temperature in the presence of an acid or a base.
There is no specific limitation to the kind of the solvent used, so long as the solvent is inert to the reaction. Examples of such solvents include aromatic hydrocarbons such as benzene, toluene, xylene, and the like, halogenated hydrocarbons such as dichloromethane, chloroform, 1,2-dichloroethane, and the like, ethers such as diethyl ether, diisopropyl ether, tetrahydrofuran, dimethyl cellosolve, diethyl cellosolve, diglyme, and the like, lower alcohols such as methanol, ethanol, propanol, tert-butanol, and the like, amides such as acetamide, dimethylacetamide, N,N-dimethylformamide, and the like, and acetic acid. Particularly preferable solvents are benzene, toluene, and acetic acid.
Either an organic acid or inorganic acid can be used for the reaction. Hydrochloric acid and sulfuric acid are typical examples given of inorganic acids. Organic acids which can be used include sulfonic acids such as methanesulfonic acid, trifluoromethanesulfonic acid, benzenesulfonic acid, p-toluenesulfonic acid, pyridine-p-toluene sulfonate; carboxylic acids such as acetic acid; and the like. Among these especially preferable are p-toluenesulfonic acid, pyridine-p-toluene sulfonate, acetic acid, and the like. Here, acetic acid can function also as a solvent.
A base to be employed in the reaction may be either an inorganic-or organic base. Given as examples of inorganic bases are hydroxides, carbonates, bicarbonates, and hydrides of alkali metal such as lithium hydroxide, sodium hydroxide, potassium hydroxide, lithium carbonate, sodium carbonate, potassium carbonate, sodium bicarbonate, potassium bicarbonate, sodium hydride, and the like. Organic bases include alkoxide of alkali metal such as sodium methoxide, sodium ethoxide, potassium tert-butoxide, and the like; tert-alkyl amines such as triethylamine, N,N-diisopropylethylamine, and the like; aromatic tertiary amines such as N,N-dimethylaniline, N,N-diethylaniline, N,N-dimethylaminopyridine, and the like; pyridine; 1,8-diazabicycloundecene; and the like. Preferred bases are potassium carbonate and triethylamine.
Some compounds of formula (3) are unstable against basic compounds. Deliberate considerations therefore must be given to the reaction conditions when a base is used. For example, measures must be taken such as carrying out the reaction at a relatively low temperature, for a shorter period of time, or under acidic conditions.
The reaction is carried out at a temperature usually of 20-150xc2x0 C., and preferably 80-120xc2x0 C. Depending on the characteristics of compound (3), however, the reaction under ice-cooling is desirable. The reaction time may be between 1 hour and 48 hours. Usually, the reaction is completed within 1-24 hours.
A typical example of performing the reaction is refluxing the reaction mixture in benzene, toluene, or acetic acid in the presence of pyridine p-toluenesulfonate.
When a group R1, R2, or R4, or their substituent is an amino group with a protective group, such a protective group can be removed by the reduction or hydrolysis with an acid or alkali.
A compound having an alkoxyl group can be converted into the corresponding hydroxyl compounds by treating them with aluminum chloride or aluminum bromide in an inert solvent such as toluene, benzene, or the like, or by heating in a solution of hydrobromic acid.
A compound having a nitro group can be converted into the corresponding amino compound by catalytic reduction using platinum, palladium, or the like.
A compound having an amino group can be converted into the corresponding hydroxyl compound via a diazonium compound by the treatment with sodium nitrite or the like in an acidic solvent at a low temperature, followed by hydrolysis of the diazonium salt.
A compound having an amino group can also be converted into the corresponding halogeno compound by the Sandmeyer reaction via diazbnium salt mentioned above. General Sandmeyer reaction conditions can be applicable to this reaction using cuprous chloride, cuprous bromide, or the like.
The compound of this invention can optionally be converted into a form of physiologically acceptable salt, e.g., a salt of an alkali metal or alkali earth metal, by using a hydroxide of these metals; or when such a compound is a basic compound such as that possessing an amino group or the like, may be converted into an inorganic or organic salt using an inorganic acid such as hydrochloric acid, sulfuric acid, phosphoric acid, or the like, or an organic acid such as formic acid, acetic acid, methanesulfonic acid or the like.
Antitumor effects of the compound of this invention thus prepared are hereinafter described by way of experimental examples.
P388 murine leukemia cells were spread over a 96-well microplate, 2.5xc3x97103 cells per well. A sample to be tested was added after 24 hours. Cells were cultivated under the conditions of 5% CO2 at 37xc2x0 C. for 3 days. 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyl-2H-tetrazolium bromide (MTT) was added 4 hours after the addition of the sample. Upon the addition bf 200 xcexcl/ml of isopropyl alcohol containing 0.04 N HCl the absorption at 540 nm was measured to determine IC50.
The results are shown in Table 1.
As shown in Table 1, the compounds of this invention have an excellent antitumor activity and a high degree of safety, and are water-soluble. They are useful as an antitumor medicine,